Polymers prepared by polymerizing conjugated dienes are useful. For example, syndiotactic 1,2-polybutadiene is a crystalline thermoplastic resin that has a stereoregular structure in which the side-chain vinyl groups are located alternately on the opposite sides in relation to the polymeric main chain. Syndiotactic 1,2-polybutadiene is a unique material that exhibits the properties of both plastics and rubber, and therefore it has many uses. For example, films, fibers, and various molded articles can be made from syndiotactic 1,2-polybutadiene. It can also be blended into and co-cured with natural or synthetic rubbers in order to improve the properties thereof. Generally, syndiotactic 1,2-polybutadiene has a melting temperature within the range of about 195.degree. C. to about 215.degree. C., but due to processability considerations, it is generally desirable for syndiotactic 1,2-polybutadiene to have a melting temperature of less than about 195.degree. C.
Various transition metal catalyst systems based on cobalt, titanium, vanadium, chromium, iron, and molybdenum for the preparation of syndiotactic 1,2-polybutadiene have been reported. The majority of these catalyst systems, however, have no practical utility because they have low catalytic activity or poor stereoselectivity, and in some cases they produce low molecular weight polymers or partially crosslinked polymers unsuitable for commercial use.
Two cobalt-based catalyst systems are known for preparing syndiotactic 1,2-polybutadiene. The first comprises a cobalt compound, a phosphine compound, an organoaluminum compound, and water. This catalyst system yields syndiotactic 1,2-polybutadiene having very low crystallinity. Also, this catalyst system develops sufficient catalytic activity only when halogenated hydrocarbon solvents are used as the polymerization medium, and halogenated solvents present toxicity problems.
The second catalyst system comprises a cobalt compound, an organoaluminum compound, and carbon disulfide. Because carbon disulfide has a low flash point, obnoxious smell, high volatility, and toxicity, it is difficult and dangerous to use and requires expensive safety measures to prevent even minimal amounts escaping into the atmosphere. Furthermore, the syndiotactic 1,2-polybutadiene produced with this cobalt catalyst system has a melting temperature of about 200-210.degree. C., which makes it difficult to process. Although the melting temperature of the syndiotactic 1,2-polybutadiene produced with this cobalt catalyst system can be reduced by employing a catalyst modifier, the use of this catalyst modifier has adverse effects on the catalyst activity and polymer yields.
Syndiotactic 1,2-polybutadiene compositions that have multiple melting temperatures are also useful. Syndiotactic 1,2-polybutadiene compositions that have a relatively high melting temperature are generally characterized by having better mechanical properties than syndiotactic 1,2-polybutadiene compositions having relatively low melting temperatures. On the other hand, syndiotactic 1,2-polybutadiene having relatively low melting temperatures are known to mix well, especially during mechanical kneading with elastomers. Syndiotactic 1,2-polybutadiene compositions that have multiple melting temperatures benefit from both of these advantages.
Because conjugated diene polymers are useful, and the catalysts known heretofore in the art have many shortcomings, there is a need for improved catalyst compositions. Also, because syndiotactic 1,2-polybutadiene compositions having multiple melting temperatures are useful, there is likewise a need to develop a catalyst composition and process for preparing these compositions.